From: Suzuki K Poulose <suzuki.poulose@arm.com> To: Dave Martin <Dave.Martin@arm.com> Cc: linux-arm-kernel@lists.infradead.org, mark.rutland@arm.com, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, will@kernel.org Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability Date: Fri, 11 Oct 2019 13:13:18 +0100 [thread overview] Message-ID: <4ba5c423-4e2a-d810-cd36-32a16ad42c91@arm.com> (raw) In-Reply-To: <20191011113620.GG27757@arm.com> Hi Dave On 11/10/2019 12:36, Dave Martin wrote: > On Thu, Oct 10, 2019 at 06:15:15PM +0100, Suzuki K Poulose wrote: >> The NO_FPSIMD capability is defined with scope SYSTEM, which implies >> that the "absence" of FP/SIMD on at least one CPU is detected only >> after all the SMP CPUs are brought up. However, we use the status >> of this capability for every context switch. So, let us change >> the scop to LOCAL_CPU to allow the detection of this capability >> as and when the first CPU without FP is brought up. >> >> Also, the current type allows hotplugged CPU to be brought up without >> FP/SIMD when all the current CPUs have FP/SIMD and we have the userspace >> up. Fix both of these issues by changing the capability to >> BOOT_RESTRICTED_LOCAL_CPU_FEATURE. >> >> Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD") >> Cc: Will Deacon <will@kernel.org> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Catalin Marinas <catalin.marinas@arm.com> >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com> >> --- >> arch/arm64/kernel/cpufeature.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >> index 9323bcc40a58..0f9eace6c64b 100644 >> --- a/arch/arm64/kernel/cpufeature.c >> +++ b/arch/arm64/kernel/cpufeature.c >> @@ -1361,7 +1361,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = { >> { >> /* FP/SIMD is not implemented */ >> .capability = ARM64_HAS_NO_FPSIMD, >> - .type = ARM64_CPUCAP_SYSTEM_FEATURE, >> + .type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE, > > ARM64_HAS_NO_FPSIMD is really a disability, not a capability. > > Although we have other things that smell like this (CPU errata for > example), I wonder whether inverting the meaning in the case would > make the situation easier to understand. Yes, it is indeed a disability, more on that below. > > So, we'd have ARM64_HAS_FPSIMD, with a minimum (signed) feature field > value of 0. Then this just looks like an ARM64_CPUCAP_SYSTEM_FEATURE > IIUC. We'd just need to invert the sense of the check in > system_supports_fpsimd(). This is particularly something we want to avoid with this patch. We want to make sure that we have the up-to-date status of the disability right when it happens. i.e, a CPU without FP/SIMD is brought up. With SYSTEM_FEATURE you have to wait until we bring all the CPUs up. Also, for HAS_FPSIMD, you must wait until all the CPUs are up, unlike the negated capability. > >> .min_field_value = 0, > > (Does .min_field_value == 0 make sense, or is it even used? I thought > only the default has_cpuid_feature() match logic uses that.) True, it is not used for this particular case. Cheers Suzuki
WARNING: multiple messages have this Message-ID (diff)
From: Suzuki K Poulose <suzuki.poulose@arm.com> To: Dave Martin <Dave.Martin@arm.com> Cc: mark.rutland@arm.com, catalin.marinas@arm.com, will@kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability Date: Fri, 11 Oct 2019 13:13:18 +0100 [thread overview] Message-ID: <4ba5c423-4e2a-d810-cd36-32a16ad42c91@arm.com> (raw) In-Reply-To: <20191011113620.GG27757@arm.com> Hi Dave On 11/10/2019 12:36, Dave Martin wrote: > On Thu, Oct 10, 2019 at 06:15:15PM +0100, Suzuki K Poulose wrote: >> The NO_FPSIMD capability is defined with scope SYSTEM, which implies >> that the "absence" of FP/SIMD on at least one CPU is detected only >> after all the SMP CPUs are brought up. However, we use the status >> of this capability for every context switch. So, let us change >> the scop to LOCAL_CPU to allow the detection of this capability >> as and when the first CPU without FP is brought up. >> >> Also, the current type allows hotplugged CPU to be brought up without >> FP/SIMD when all the current CPUs have FP/SIMD and we have the userspace >> up. Fix both of these issues by changing the capability to >> BOOT_RESTRICTED_LOCAL_CPU_FEATURE. >> >> Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD") >> Cc: Will Deacon <will@kernel.org> >> Cc: Mark Rutland <mark.rutland@arm.com> >> Cc: Catalin Marinas <catalin.marinas@arm.com> >> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com> >> --- >> arch/arm64/kernel/cpufeature.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c >> index 9323bcc40a58..0f9eace6c64b 100644 >> --- a/arch/arm64/kernel/cpufeature.c >> +++ b/arch/arm64/kernel/cpufeature.c >> @@ -1361,7 +1361,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = { >> { >> /* FP/SIMD is not implemented */ >> .capability = ARM64_HAS_NO_FPSIMD, >> - .type = ARM64_CPUCAP_SYSTEM_FEATURE, >> + .type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE, > > ARM64_HAS_NO_FPSIMD is really a disability, not a capability. > > Although we have other things that smell like this (CPU errata for > example), I wonder whether inverting the meaning in the case would > make the situation easier to understand. Yes, it is indeed a disability, more on that below. > > So, we'd have ARM64_HAS_FPSIMD, with a minimum (signed) feature field > value of 0. Then this just looks like an ARM64_CPUCAP_SYSTEM_FEATURE > IIUC. We'd just need to invert the sense of the check in > system_supports_fpsimd(). This is particularly something we want to avoid with this patch. We want to make sure that we have the up-to-date status of the disability right when it happens. i.e, a CPU without FP/SIMD is brought up. With SYSTEM_FEATURE you have to wait until we bring all the CPUs up. Also, for HAS_FPSIMD, you must wait until all the CPUs are up, unlike the negated capability. > >> .min_field_value = 0, > > (Does .min_field_value == 0 make sense, or is it even used? I thought > only the default has_cpuid_feature() match logic uses that.) True, it is not used for this particular case. Cheers Suzuki _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2019-10-11 12:13 UTC|newest] Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-10-10 17:15 [PATCH 0/3] arm64: Fix support for systems without FP/SIMD Suzuki K Poulose 2019-10-10 17:15 ` Suzuki K Poulose 2019-10-10 17:15 ` [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability Suzuki K Poulose 2019-10-10 17:15 ` Suzuki K Poulose 2019-10-11 11:36 ` Dave Martin 2019-10-11 11:36 ` Dave Martin 2019-10-11 12:13 ` Suzuki K Poulose [this message] 2019-10-11 12:13 ` Suzuki K Poulose 2019-10-11 14:21 ` Dave Martin 2019-10-11 14:21 ` Dave Martin 2019-10-11 17:28 ` Suzuki K Poulose 2019-10-11 17:28 ` Suzuki K Poulose 2019-10-14 14:52 ` Dave Martin 2019-10-14 14:52 ` Dave Martin 2019-10-14 15:45 ` Suzuki K Poulose 2019-10-14 15:45 ` Suzuki K Poulose 2019-10-14 15:50 ` Dave P Martin 2019-10-14 15:50 ` Dave P Martin 2019-10-14 16:57 ` Ard Biesheuvel 2019-10-14 16:57 ` Ard Biesheuvel 2019-10-15 9:44 ` Suzuki K Poulose 2019-10-15 9:44 ` Suzuki K Poulose 2019-10-15 9:52 ` Ard Biesheuvel 2019-10-15 9:52 ` Ard Biesheuvel 2019-10-15 10:24 ` Dave Martin 2019-10-15 10:24 ` Dave Martin 2019-10-15 10:30 ` Ard Biesheuvel 2019-10-15 10:30 ` Ard Biesheuvel 2019-10-15 13:03 ` Suzuki K Poulose 2019-10-15 13:03 ` Suzuki K Poulose 2019-10-15 13:11 ` Ard Biesheuvel 2019-10-15 13:11 ` Ard Biesheuvel 2019-10-15 14:05 ` Dave Martin 2019-10-15 14:05 ` Dave Martin 2019-10-10 17:15 ` [PATCH 2/3] arm64: nofpsmid: Clear TIF_FOREIGN_FPSTATE flag for early tasks Suzuki K Poulose 2019-10-10 17:15 ` Suzuki K Poulose 2019-10-11 11:26 ` Dave Martin 2019-10-11 11:26 ` Dave Martin 2019-10-17 12:42 ` Suzuki K Poulose 2019-10-17 12:42 ` Suzuki K Poulose 2019-10-17 16:09 ` Dave Martin 2019-10-17 16:09 ` Dave Martin 2019-10-10 17:15 ` [PATCH 3/3] arm64: cpufeature: Set the FP/SIMD compat HWCAP bits properly Suzuki K Poulose 2019-10-10 17:15 ` Suzuki K Poulose 2019-10-17 0:06 ` [PATCH 0/3] arm64: Fix support for systems without FP/SIMD Will Deacon 2019-10-17 0:06 ` Will Deacon
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